Tens of thousands of protesters marched in Mexico City on Saturday to protest against Enrique Peña Nieto's apparent win in the country's presidential election, accusing his party of buying votes and paying TV networks for support.

"The fraud was carried out before (the election), buying votes, tricking the people," said Gabriel Petatan Garcia, a geography student who carried a sign in Finnish. Protesters also carried signs in English, Japanese, French, German and other languages to call the attention of the international press.

"The PRI threatens many people and buys others with a couple of tacos," said Manuel Ocegueda, a 43-year-old shop worker at the rally.

Peña Nieto, a youthful 45-year-old married to a soap star, won last Sunday's election by 6.6 percentage points, according to the official count, bringing the PRI back to power after 12 years in opposition. The party had ruled Mexico for 71 consecutive years, with what critics say was the help of corruption, patronage and vote fraud.

PRI officials deny buying votes and say the elections were free and fair.

The final count had Peña Nieto with 38.21% support, leftist Andrés Manuel López Obrador of the Democratic Revolution party with 31.59%, and Josefina Vazquez Mota of the conservative National Action party with 25.41%. The small New Alliance Party got 2.29%.

The final count will be certified in September by the Federal Electoral Tribunal. The tribunal has declined to overturn previously contested elections, including a 2006 presidential vote that was far closer than last Sunday's.

In the weeks before the latest polls a student-led movement, Soy132, mobilised demonstrations and online protests against his links to the media giant Televisa, saying that both manipulate public opinion and state institutions in malign synergy.

Accusations of vote-buying began surfacing in June, but sharpened later when people rushed to grocery stores on the outskirts of Mexico City to redeem pre-paid gift cards worth about 100 pesos (£4.50). Many said they got the cards from PRI supporters before the elections.

López Obrador said millions of voters had received either pre-paid cards, cash, groceries, construction materials or appliances.

Some demonstrators covered the heads of statues with plastic shopping bags from Soriana, the supermarket chain where the gift cards were redeemable. "We have to come out in the streets to denounce that the PRI bought votes, and there were people who sold them," said a 32-year-old psychologist, Raquel Ruiz.

Some protesters said overturning the election result would be difficult, while others thought there were judicial means to prevent Peña Nieto from assuming the presidency.

López Obrador said he would file a formal legal challenge to the vote count in electoral courts based on the allegation that PRI vote-buying influenced millions of votes.

Simply giving away such gifts is not illegal under Mexican electoral law, as long as the expense is reported to electoral authorities. Giving gifts to influence votes is a crime, though it is not generally viewed as grounds for overturning an election.

Leonardo Valdés, the president of the Federal Electoral Institute, said he did not see any grounds for overturning the results but that an investigation into the gift cards had been launched.

The PRI spokesman, Eduardo Sánchez, said last week the gift-card event had been "a theatrical representation" mounted by the left. He claimed supporters of López Obrador took hundreds of people to the shops, dressed them in PRI T-shirts, gave them gift cards, emptied shelves to create an appearance of panic buying, and brought TV cameras in to give the false impression that the PRI had given out the cards.